The present invention relates to gel for a radiation dosimeter useful for measuring accurately the spatial dose distribution at the position of a human body or an animal body irradiated before performing the radiation therapy and a radiation dosimeter which uses the same.
In the radiation therapy, the effective dose with the target regions of cancer is required. And, the minimum dose which does not cause the radiation injury to a normal tissue in the surroundings of the focus is required. Therefore, a variety of radiation therapy methods have been developed. Especially, stereotactic radiotherapy or intensity modulated radiation therapy using a gamma knife or a cyber knife is becoming increasingly popular. Recently, the radiation therapy technology by proton beam irradiation or heavy particle beam irradiation has been used in clinical practice.
In general, when the radiation therapy is performed, the site and the shape etc. of cancer areas are specified by using X-ray computed tomography (X-ray CT) scanner, magnetic resonance imaging (MRI), etc. beforehand. And, the radiation therapy planning such as dose, an irradiation method, etc. is conducted on the basis of the obtained information. Though three-dimensional dose distribution is calculated by using a radiation therapy plan device, and forecasted, the conventional ionization chamber dosimeter or solid state dosimeter, etc. cannot obtain excluding the dose distribution in a point or plane (two-dimension). That is, it is difficult to actually measure a continuous spatial dose distribution (three-dimension) so far. Being paid attention as a dosimeter to measure this spatial dose distribution is a gel dosimeter. Therefore, the gel dosimeter aimed at the establishment of a highly accurate dose evaluation system is actively researched and developed in recent years (Patent Literatures 1 and 2, and Non-Patent Literatures 1 to 3).
Fricke gel dosimeter or a polymer gel dosimeter is reported as a gel dosimeter by which the three-dimensional dose distribution can be measured. Fricke gel dosimeter is composed of the gel which contains the solution of Fricke dosimeter known as a liquid chemical dosimeter (the solution which contains ferrous sulfate). This dosimeter uses the principle that the oxidation reaction (color) of iron from bivalent to trivalent metal according to the irradiation increases in proportion to the absorbed dose. Though the chemical composition adjustment etc. is devised to raise the chemical yield G-value of ferrous ion (trivalent), there is a problem that ferrous ion (trivalent) in the gel diffuses over time, and thus the dose distribution is instable with time. While, in the polymer gel dosimeter, monomer is distributed in the gel, and because polymer is generated in proportion to dose, the dose can be estimated from an amount of the polymer generated (degree of cloudiness). The polymer gel dosimeter has excellent features that it is difficult for the generated polymer to diffuse in the gel, and the cloudy part is stable with time and seems to be the floatage in the transparent gel.
Higher-sensitivity gel dosimeter by which the irradiation position and accuracy can be accurately evaluated has been requested to manage appropriately the quality of the radiation therapy which has been complicated. Therefore, the kind of monomer, the chemical composition adjustment, and the addition of the scavenger for dissolved oxygen, free radicals, etc. are studied in various ways so as to get cloudy even in lower dose (Non-Patent Literatures 1 and 2).
Polymer of N-isopropyl acrylamide (NIPA) is known as the compound which exhibits phase transition behavior (cloudy-transparent state) in response to external temperature change, used in the present invention like a compound described later. And, there is a report that this NIPA is used as monomer for a gel dosimeter (Non-Patent Literature 4). However, the monomer described in Non-Patent Literature 4 is used only as one of monomers to provide a low toxic effect. Therefore, this reference does not provide means for solving the above-mentioned problem of the present invention.    [Patent Literature 1] JP 2002-214354A    [Patent Literature 2] U.S. Pat. No. 5,321,357    [Non-Patent Literature 1] Journal of Physics: Conference Series, 56, 23-34 (2006).    [Non-Patent Literature 2] Journal of Physics: Conference Series, 56, 35-44 (2006).    [Non-Patent Literature 3] Journal of Applied Clinical Medical Physics, 7, 13-21 (2006).    [Non-Patent Literature 4] Physics in Medicine and Biology, 51, 3301-3314 (2006)